1. Field of the Invention
The present invention relates to a surface acoustic wave device for use in, for example, delay lines, filters, and other electronic components, and to a communication device including a surface acoustic wave device.
2. Description of the Related Art
Since electronic devices have become smaller in size and lighter in weight in recent years, there has been an increasing demand for electronic devices with multiple functions. With such a background, there has also been an increasing demand for surface acoustic wave filters (hereinafter referred to as xe2x80x9cSAW filtersxe2x80x9d) incorporated in surface acoustic wave devices used in communication devices, such as cellular phones, to have an unbalanced-to-balanced conversion function so that the SAW filters can be directly connected to an IC which operates by using a balanced input, and active research has been intensively carried out in this area.
In particular, in a SAW filter formed of a resonator filter, having an unbalanced-to-balanced conversion function, the amplitude difference and the phase difference on the balanced side are important characteristics (hereinafter referred to as xe2x80x9cbalanced characteristicsxe2x80x9d), and it is necessary that the amplitude difference be 0 dB and the phase difference be 180xc2x0 on the balanced side. However, in practice, since the tendencies of the balanced characteristics also differ according to the structure of the SAW filter, it is not possible to accurately achieve an amplitude difference of 0 dB and a phase difference of 180xc2x0, and improvement of the balanced characteristics with respect to the structure of the SAW filter has become an important objective in such devices.
For the surface acoustic wave device having an unbalanced-to-balanced conversion function, there are various structures depending on the type and purpose of an IC. For example, for the surface acoustic wave device in which the matching impedance on the balanced side is approximately four times as large as the matching impedance on the unbalanced side, the structure such as that shown in FIG. 12 is widely used.
A surface acoustic wave device shown in FIG. 12 is configured in such a manner that a resonator filter 100 and a resonator filter 101, which is 180xc2x0 out of phase with the resonator filter 100, are provided on a piezoelectric substrate (not shown).
The resonator filter 100 is provided with a comb-shaped electrode (an interdigital transducer, hereinafter abbreviated as an xe2x80x9cIDTxe2x80x9d) 101, and IDTs 102 and 103 are arranged on the right and left sides (the right and left direction along the propagation direction of the surface acoustic wave) of the IDT 101. Furthermore, in the resonator filter 100, reflectors 104 and 105 are arranged so as to sandwich the IDTs 101, 102, and 103 from the right and left (the right and left direction along the propagation direction of the surface acoustic wave), respectively.
The IDT is formed from a metal thin-film of aluminum, etc., and functions as a surface acoustic wave conversion section which converts an input electrical signal (AC) into surface acoustic waves (SAW energy) so that the surface acoustic waves propagate on the piezoelectric substrate, and which converts the propagated surface acoustic waves into electrical signals and outputs them. The reflector is used to reflect the propagated surface acoustic waves back in the direction from which they came in order to improve the conversion efficiency.
In such an IDT, it is possible to set signal conversion characteristics and a passband by setting the length and the width of each comb-shaped electrode finger, the spacing between adjacent comb-shaped electrode fingers, and the finger overlap indicating the opposing length in an interdigitated state between mutual comb-shaped electrode fingers. In the reflector, reflection characteristics can be set by adjusting the width of each reflector electrode finger and the spacing between fingers.
A resonator filter 110 is provided with an IDT 111 in which the hot side (signal side) and the ground side of the electrode finger are reversed with respect to the IDT 101 in the resonator filter 100 so that the phase differs by 180xc2x0 with respect to the resonator filter 100. IDTs 112 and 113 are provided on the right and left (the right and left direction along the propagation direction of the surface acoustic wave) of the IDT 111. Furthermore, in the resonator filter 110, reflectors 114 and 115 for reflecting surface acoustic waves in order to improve the conversion efficiency are arranged so as to sandwich the IDTs 111, 112, and 113 from the right and left, respectively.
More specifically, there are provided an unbalanced signal terminal 170 to which the IDTs 102 and 103 in the resonator filter 100 and the IDTs 112 and 113 in the resonator filter 110 are connected in parallel, and balanced signal terminals 180 and 190 which are connected in series to the IDTs 101 and 1111, respectively. That is, one side of each of the resonator filters 100 and 110 which are 180xc2x0 out of phase with each other is connected, and a surface acoustic wave device having an unbalanced-to-balanced conversion function is configured in such a manner that the connected unbalanced signal terminal 170 is an unbalanced terminal and the balanced signal terminals 180 and 190 which are not connected are balanced terminals. For this surface acoustic wave device, high attenuation and sharpness of attenuation outside the passband are required.
Therefore, as shown in FIG. 13, in each of the terminals 170, 180, and 190 of the surface acoustic wave device shown in FIG. 12, by arranging in series trapping resonators 130, 140, and 150 having resonators 131, 141, and 151, respectively, high attenuation and sharpness of attenuation can be obtained outside the passband.
However, the above-described structure has factors that deteriorate balanced characteristics. For example, since the hot side and the ground side of the electrode finger in the central IDT 111 of the resonator filter 110 are reversed with respect to those of the IDT 101 in the resonator filter 100 so that the phase differs by 180xc2x0 with respect to the resonator filter 100, the number of the hot side and the ground side of the resonator filter 100 and that of the resonator filter 110 differ, and the ground side and the hot side are aligned in the IDT-IDT interface, causing an unwanted electric-field to be generated. As one of the countermeasures to improve these balanced characteristics, there is a method of grounding the hot electrode finger, but still a problem remains in the balanced characteristics.
For the surface acoustic wave device shown in FIG. 12, an ideal state is that only the phase difference is 180xc2x0 between the resonator filter 100 and the resonator filter 110 in the vicinity of the passband. However, after all, for the reasons described above, in practice, the impedance, etc., also differs. As a result, the balanced characteristics deviate from the ideal amplitude difference of 0 dB and the ideal phase difference of 180xc2x0. Such an inconvenience is a problem which inherently occurs in a surface acoustic wave device having an unbalanced-to-balanced conversion function, including 2-system filter sections which are 180xc2x0 out of phase with each other. Furthermore, there is no effective practical method for solving such problems when the balanced characteristics experience these problems.
In order to overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave device having an unbalanced-to-balanced conversion function, in which balanced characteristics are greatly improved.
According to a preferred embodiment of the present invention, a surface acoustic wave device includes a first surface acoustic wave element having a plurality of interdigital transducers arranged along the propagation direction of a surface acoustic wave, and a second surface acoustic wave element having a plurality of interdigital transducers arranged along the propagation direction of a surface acoustic wave, the surface acoustic wave device having an unbalanced-to-balanced conversion function as a result of the phase of the second surface acoustic wave element being reversed by 180xc2x0 with respect to the first surface acoustic wave element, a first resonator which is connected in series to the unbalanced side of the first surface acoustic wave element, and a second resonator which is connected in series to the unbalanced side of the second surface acoustic wave element, and the first resonator and the second resonator have different design parameters. The first resonator and the second resonator are preferably surface acoustic wave resonators including at least one interdigital transducer and reflectors arranged so as to sandwich the at least one interdigital transducer.
According to the above-described arrangement, since the first resonator and the second resonator are designed in accordance with the characteristics of the first surface acoustic wave element and the second surface acoustic wave element, respectively, balanced characteristics of the surface acoustic wave device in unbalanced-to-balanced conversion can be improved by adjusting the design parameters of the first and second resonators.
In the surface acoustic wave device of preferred embodiments of the present invention, in addition to the above-described arrangement, the design parameters may be the number of electrode fingers of the reflectors and/or the interdigital transducer in the first resonator and the second resonator.
According to the above-described arrangement, balanced characteristics of the output on the balanced side can be improved by changing the number of electrode fingers in each resonator.
In the surface acoustic wave device of preferred embodiments of the present invention, in addition to the above-described arrangement, the design parameters are preferably the amount of finger overlaps in the first resonator and the second resonator.
According to the above-described arrangement, balanced characteristics of the output on the balanced side can be improved by changing the finger overlap in each resonator.
In the surface acoustic wave device of preferred embodiments of the present invention, in addition to the above-described arrangement, the design parameters may be duties in the first resonator and the second resonator. Furthermore, when the duties of reflectors and/or an interdigital transducer in the first resonator and the second resonator are denoted as x and y, respectively, the relationship of 0 less than |xxe2x88x92y|xe2x89xa60.05 is preferably satisfied.
According to the above-described arrangement, balanced characteristics of the output on the balanced side can be improved by changing the duty in each resonator.
According to the above-described arrangement, balanced characteristics of the output on the balanced side can be improved by changing the ratio of the first resonator to the second resonator.
In the surface acoustic wave device of preferred embodiments of the present invention, in addition to the above-described arrangement, the design parameters may be the distances between the centers of the outermost electrode fingers of the reflectors and the interdigital transducer in the first resonator and the second resonator. Furthermore, when the wavelength determined by the structure of the interdigital transducer of the surface acoustic wave element is denoted as xcex, and the distances between the centers of the outermost electrode fingers of the reflectors and the interdigital transducer in the first resonator and the second resonator are denoted as Xxcex and Yxcex, respectively, the relationship of (0+0.5 n)xcex less than |Xxe2x88x92Y|xcexxe2x89xa6(0.18+0.5 n)xcex, where n=0, 1, 2 . . . , is preferably satisfied.
According to the above-described arrangement, since the distances between the centers of the outermost electrode fingers of the first resonator and the second resonator differ, the amplitude and phase characteristics of the first resonator and the second resonator differ, and the deviation of the degree of balance at higher frequencies of the passband in the first surface acoustic wave element and the second surface acoustic wave element can be corrected. Therefore, it is possible to obtain a surface acoustic wave device having a large common-mode attenuation at higher frequencies of the passband.
In the surface acoustic wave device of preferred embodiments of the present invention, in addition to the above-described arrangement, the design parameters may be the pitch ratios of the reflectors and the interdigital transducer in the first resonator and the second resonator. Furthermore, when the pitch ratios of the reflectors and the interdigital transducer (pitch of the interdigital transducer/pitch of the reflectors) in the first resonator and the second resonator are denoted as a and b, respectively, the relationship of 0.984xe2x89xa6a/b less than 1 is preferably satisfied.
According to the above-described arrangement, since the pitch ratio of the interdigital transducer and the reflector of the first resonator and the second resonator are different, the amplitude and phase characteristics of the first resonator and the second resonator differ, and the deviation of the degree of balance at higher frequencies of the passband in the first surface acoustic wave element and the second surface acoustic wave element can be corrected. Therefore, it is possible to obtain a surface acoustic wave device having a large common-mode attenuation at higher frequencies of the passband.
The surface acoustic wave device may be housed in a package by a face-down technique or other suitable mounting technique.
The communication device of another preferred embodiment of the present invention includes one of the above-described surface acoustic wave devices according to other preferred embodiments of the present invention.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.